This invention relates to a preprocessing method and device for speech recognition. More particularly, it relates to a preprocessing method and device for recognizing speech which is received through a transmission system, such as a telephone line network.
In an information service system which connects a telephone line network to an information service center, including a computer system, and a telephone line network, speech from an ARU (audio response unit) is employed as an output from the information service center, while a push-button signal is employed or voice is directly used as an input from a speaker to the telephone line network. The input speech from the speaker is transmitted to the information service center via the telephone line network. In the information service center, speech recognition is performed by investigating the similarity between the input speech and a standard speech signal.
The input speech having passed through the network in this manner is distorted under the influence of the transmission characteristic of the transmission system. On the other hand, the distortion characteristic of the network is not uniform, but it usually differs depending upon the various routes the information takes through the transmission system. In the speech recognition device, accordingly, input speech received from circuits having different distortion characteristics are typically compared. It is therefore necessary to eliminate the influence of the distortion or to reduce it by normalization or other means if accurate speech recognition is to be obtained.
As a method for correcting the distortion of the spectrum of the input speech dependent upon the transmission circuit to components for the problems described above, a method disclosed in U.S. patent application Ser. No. 880,951, filed by the same inventor as in the present application, has been proposed.
In this method, a predetermined section of input speech is analyzed to extract a spectrum of information in the section, and an inverse filter which has a characteristic inverse to the spectrum is provided. The input speech is then passed through the inverse filter so as to cancel the influence of the transmission characteristic of the transmission circuit on the input speech. With such a prior-art method, however, the inverse filter becomes very astable and inferiorly follows the characteristic within the speech transmission band. Another disadvantage is that it is susceptible to noise outside the speech transmission band.